At a time of sudden vehicle starts or sudden vehicle stops, excess external forces are applied to parts used in a machine construction, especially, differential gears, transmission gears, carburized toothed shafts or other gear parts. At this time, a high degree of stress is generated within a base portion of tooth of the gear part. As a result, fall or breakage of tooth may occur at the base portion of the tooth because of receiving a static bending stress. Therefore, it has been strongly demanded that the static bending strength be improved, especially for the differential gears. In the past, a case hardening steel containing about 0.2% of C according to JIS-SCr420, JIS-SCM420 or the like has been generally used for a base material (steel before carburization is applied) for the gear part as described above. This makes it possible to lower the hardness of the base material, and maintain the machinability before the carburization, for example, at the time of performing a cutting operation such as teeth cutting, which is implemented before the carburization. Then, a carburizing operation (carburizing and hardening operation, and low-temperature tempering operation at around 150° C.) is applied after the cutting operation to transform a metal structure of a surface of the carburized steel part into a tempered martensite structure (troostite structure or sorbite structure) containing about 0.8% of C. FIG. 7 is a diagram showing a relationship between a depth from the surface and Vickers hardness of the carburized steel part obtained by the processes as described above. As shown in FIG. 7, the hardness of the surface layer portion can be strengthened through the processes as described above, and hence, the high-cycle bending fatigue strength and the wear resistance of the gear part can be improved by implementing the processes as described above to the gear part.
Patent Literatures 1-3, which will be described in detail later, disclose techniques for improving the static bending strength of the carburized steel part.
Patent Literature 1 discloses a carburized steel part manufactured from a base material containing chemical components of 0.1-0.3 wt % of C, 0.35-1.1 wt % of Mn, 0.1-1.1 wt % of Cr, 0.6-1.7 wt % of Mn+Cr, and 0.001-0.005 wt % of B, in which the amount of C in a surface portion of a carburized and hardened layer is 0.6-1.1 wt %, and a troostite area fraction in the carburized and hardened layer is 5-50%.
Patent Literature 2 discloses a carburized steel part manufactured from a base material containing chemical components of 0.1-0.3 wt % of C, 0.5-1.3 wt % of Mn, 0.1-1.1 wt % of Cr, 0.9-1.9 wt % of Mn+Cr, and 0.001-0.005 wt % of B, in which the amount of C in a surface portion of a carburized and hardened layer is 0.6-1.1 wt %, and a troostite area fraction in the carburized and hardened layer is 5-50%.
Patent Literature 3 discloses a method in which a carburizing operation is applied to a formed product made by using alloy steel containing 0.5% or more of Ni, and a region from a surface of the carburized formed product up to a depth of 20 micrometers or more is removed by electrolytic polishing and the like.